The broad objectives of this program are to explore the molecular basis and physiological functions of ion transport in microorganisms. There is now strong evidence that in bacteria the linkage between metabolism and transport is usually indirect, effected by a circulation of protons across the cytoplasmic membrane (chemiosmotic theory). At the same time we have come to recognize a second major class of transport systems that are energized by a phosphoryl donor, possibly ATP. Indeed, some important systems including those for K plus, Na plus and Pi minus require both ATP and the proton circulation. We are studying the molecular basis of ATP-linked transport, its interaction with the proton circulation and its significance in the economy of microbial cells. Two specific lines are being pursued. The first continues ongoing research on ion transport in intact cells and membrane vesicles of Streptococcus faecalis, a bacterium that offers both technical and conceptual advantages. The second line, recently initiated, is concerned with the mechanism of ion movements in the fungus Blastocladiella emersonii and their relationship to the electrical properties, growth and development of this remarkable organism.